1. Field of The Invention
This invention relates to wireless communications and, more particularly, to a measurement radio system in a wireless communications station for providing operating information to traffic radios.
2. Description of Related Art
FIG. 1 depicts a schematic diagram of a portion of a typical wireless telecommunications system, which provides wireless telecommunications service to a number of wireless terminals (e.g., wireless terminals 101-1 through 101-3) that are situated within a geographic region. The heart of a typical wireless telecommunications system is a Mobile Switching Center (xe2x80x9cMSCxe2x80x9d) 120, which might be known also as a Wireless Switching Center (xe2x80x9cWSCxe2x80x9d) or a Mobile Telephone Switching Office (xe2x80x9cMTSOxe2x80x9d). Typically, the Mobile Switching Center 120 is connected to a plurality of base stations (e.g., base stations 103-1 through 103-5) that are dispersed throughout the geographic area serviced by the system and to the local and long-distance telephone offices (e.g., local-office 130, local-office 138 and toll-office 140). The Mobile Switching Center 120 is responsible for, among other things, establishing and maintaining calls between the wireless terminals and calls between a wireless terminal and a wireline terminal (e.g., wireline terminal 150), which wireline terminal is connected to the Mobile Switching Center 120 via the local and/or long-distance networks.
The geographic area serviced by a wireless telecommunications system is divided into spatially distinct areas called xe2x80x9ccells.xe2x80x9d As depicted in FIG. 1, each cell is schematically represented by one hexagon in a honeycomb pattern; in practice, however, each cell has an irregular shape that depends on the topography of the terrain surrounding the cell. Typically, each cell contains a base station, which comprises the radios and antennas that the base station uses to communicate with the wireless terminals in that cell and also comprises the transmission equipment that the base station uses to communicate with Mobile Switching Center 120. For example, when wireless terminal 101-1 desires to communicate with wireless terminal 101-2, wireless terminal 101-1 transmits the desired information to base station 103-1, which relays the information to Mobile Switching Center 120. Upon receipt of the information, and with the knowledge that it is intended for wireless terminal 101-2, Mobile Switching Center 120 then returns the information back to base station 103-1, which relays the information, via radio, to wireless terminal 101-2.
FIG. 2 depicts a block diagram of a first base station architecture in the prior art, which comprises one or more radios that are capable of transmitting outgoing signals via a transmit antenna (xe2x80x9cTXxe2x80x9d) and receiving incoming signals via a receive antenna (xe2x80x9cRxxe2x80x9d). According to this architecture, there is only one transmit antenna per cell that transmits omni-directionally and only one receive antenna per cell that receives omni-directionally. Each radio in this architecture receives one incoming carrier signal via the receive antenna and demodulates that carrier signal into one or more baseband signals in accordance with the particular access scheme employed (e.g., frequency-division multiple access, time-division multiple access, code-division multiple-access, etc.). The incoming baseband signals are then transmitted to wireless switching center 120. Analogously, outgoing baseband signals from wireless switching center 120 are modulated by the radio in accordance with the particular multiplexing scheme employed (e.g., frequency-division multiplexing, time-division multiplexing, code-division multiplexing, etc.) for transmission via the transmission antenna.
When wireless telecommunications system 100 is a terrestrial system, in contrast to a satellite-based system, the quality and availability of service is subject to the idiosyncrasies of the terrain surrounding the system. For example, when the topography of the terrain is hilly or mountainous, or when objects such as buildings or trees are present, a signal transmitted by a wireless terminal can be absorbed or reflected such that the signal quality is not uniform at the base station. As such, many independent paths result from the scattering and reflection of a signal between the many objects that lie between and around the mobile terminal and the base station. The scattering and reflection of the signal creates many different xe2x80x9ccopiesxe2x80x9d of the transmitted signal (xe2x80x9cmultipath signalsxe2x80x9d) arriving at the receive antenna of the base station with various amounts of delay, phase shift and attenuation. As a result, the signal received at the base station from the mobile unit is made up of the sum of many signals, each traveling over a separate path. Since these path lengths are not equal, the information carried over the radio link will experience a spread in delay as it travels between the base station and the mobile station. The amount of time dispersion between the earliest received copy of the transmitted signal and the latest arriving copy having a signal strength above a certain level is often referred to as delay spread. Delay spread can cause intersymbol interference (ISI). In addition to delay spread, the same multipath environment causes severe local variations in the received signal strength as the multipath signals are added constructively and destructively at the receive antenna of the base station. This phenomenon is widely known as multipath fading or fast fading or Rayleigh fading.
FIG. 3 depicts a block diagram of a second base station architecture in the prior art, which supports a technique known as N-way receive diversity to mitigate the effects of multipath fading. The base station architecture depicted in FIG. 3 comprises one or more radios that are capable of transmitting outgoing signals via a single transmit antenna, as in the architecture of FIG. 2, but also comprises N spatially-separate receive antennas (xe2x80x9cRx1xe2x80x9d through xe2x80x9cRxNxe2x80x9d). Because multipath fading is a localized phenomenon, it is highly unlikely that all of the spatially separated receive antennas will experience multipath fading at the same time. Therefore, if an incoming signal is weak at one receive antenna, it is likely to be satisfactory at one of the others. As is well-known in the prior art, a diversity combiner associated with the radios can combine N incoming signals, each from one of N receive antennas, using various techniques (e.g., selection diversity, equal gain combining diversity, maximum ratio combining diversity, etc.) to improve the reception of an incoming signal.
FIG. 4 depicts a block diagram of a third base station architecture in the prior art, which supports a technique for increasing the traffic capacity of the telecommunications system. This technique is known as xe2x80x9cbase station sectorization.xe2x80x9d In accordance with base station sectorization, the cell serviced by a base station is subdivided into M tessellated pie-slices, each of which comprises a 360xc2x0/M sector whose focus is at the base station. The base station architecture in FIG. 4 comprises M sets of radios and associated transmit and receive antennas, as shown, each of which operates independently of the others, except that the transmit and receive antennas associated with each sector are generally implemented so as to principally transmit into and receive from that sector.
The architecture in FIG. 4 is, however, disadvantageous because it requires more radios than necessary to support a given traffic capacity, which unnecessarily increases the cost of the base station. The same average traffic capacity can be accommodated with fewer radios if they are pooled, as depicted in FIG. 5. FIG. 5 depicts a block diagram of a fourth base station architecture in the prior art, which supports receive diversity, sectorization, and radio pooling. The architecture comprises: a plurality of radios 501-1 through 501-Z, sniffer radio 502, switch matrix 503, and M sets of transmit and receive antennas 504-1 through 504-M, interconnected as shown. In accordance with this architecture, sniffer radio 502 scans all of the potential sectors and channels in search of incoming signals. When sniffer radio 502 detects an incoming signal from a given sector, it directly controls the switch matrix 503 to route the incoming signals from that sector to an appropriate radio and to route the outgoing signals from that radio to the same sector. Because any radio can receive from and transmit to any sector, this architecture requires fewer radios to support the same average traffic capacity as the architecture in FIG. 4. Because the sniffer radio 502 acts as a master radio and is responsible for switching to a set of antennas for the traffic radios 501-1-501Z, the sniffer radio can become a bottleneck in the operation of the system. For example, if a mobile unit engaged in a call with the base station moves from one sector to another, but the sniffer radio 502 is late in determining that the mobile unit has moved into another sector which requires a different set of antennas, the call might be dropped to the base station. Furthermore, the baseband processing of the radios in the architectures of FIG. 1-5 have limited ability beyond those described when servicing a dedicated voice/data channel because of cost, size, and processor performance.
The present invention involves a measurement radio system which uses a measurement radio to scan active channels of a base station and produce operating information for the traffic radios servicing the active channels. The measurement radio can produce operating information, such as signal strength, bit error rate (BER), frame error rate (FER) and signal to interference ratio (C/I), which is used to determine whether to change the manner in which the traffic radio is servicing the active channel. For example, if the measurement radio can switch between different sets of antennas, the measurement radio can scan an active voice/data channel using a different set of antennas than the traffic radio is using to service the active channel and determine operating information related to the signal received over the active channel using the different set of antennas. The traffic radio can use the operating information to determine whether to hand off the active channel to the different set of antennas.
The measurement radio can determine operating information, such as operating coefficients, parameters or settings, to change how the traffic radio services the active channel. For example, if the active channel is to be handed off to a different set of antennas, the measurement radio can provide automatic gain control (AGC) settings for the traffic radio to use in servicing the active channel using the different set of antennas. As such, the measurement radio can provide seamless handoffs between different sets of antennas because the traffic radio already has the operating settings corresponding to the active call on the different set of antennas. The measurement radio can determine other operating information to change the manner in which the traffic radio services the active channel. For example, the measurement radio can provide filter coefficients, equalizer coefficients, or operating coefficients for the receive algorithms of the traffic radios, such as coefficients related to the speed of the mobile terminal on an active channel. The measurement radio can also provide operating information related to the active channel conditions, such as information related to delay spread conditions and the speed of the mobile terminal. In certain embodiments, the delay spread information can determine whether the traffic radio uses differential detection or equalization to service the active channel.
The measurement radio can also scan the transmissions of the traffic radios. In response to the transmissions of a traffic radio, the measurement radio can send operating information to the traffic radio that determines how the traffic radio transmits over the active channel. For example, the measurement radio could send operating coefficients or parameter to the traffic radios to change the respective phase, amplitude and/or output power of individual symbols transmitted over respective active voice/data channels of the entire base station.
The measurement radio can provide operating information to the traffic radios as parameters or coefficients updated in dedicated storage locations corresponding to the respective traffic radios and/or the respective active channels or sent over a radio communication bus to dedicated addresses or at a dedicated time slot for the traffic radios and/or the respective active channels. The measurement radio can periodically provide operating information to the traffic radio or in response to a request by the traffic radio or the MSC. Alternatively, the measurement radio can produce the operating information to the MSC or a base staton controller which determines the appropriate operating information to produce to the traffic radio.